1. Technical Field
The invention relates to an electro-acoustical device and, more particularly, to a horn loaded loudspeaker for reproducing low frequency audible sound at high power output levels from a plurality of electric-acoustic transducers having relatively small diaphragms and enclosed in a compact, preferably portable, enclosure.
2. Description of the Problem
The reproduction of low frequency audible sound, with high fidelity and at high intensity levels, poses a number of challenges. To do so from a small, energy efficient package, portable enough to be moved and suitable for open air use is especially difficult. Generally, high output, high efficiency, low frequency loudspeakers have been built around a horn. A horn is in effect an acoustic transformer, allowing the designer to obtain the output performance of a larger area diaphragm than that possessed by the acoustic driver. At the same time cone/diaphragm resonance issues that exist with direct radiator devices are minimized. Increasing the effective diaphragm area renders radiation impedance increasingly resistive with the result that increasing power may be absorbed at the desired low frequencies. However, increasing acoustic power output from most horn designs has required increasing diaphragm piston travel in order to move the required volume velocity of air. Piston travel becomes an important limiting factor relating to the amount of power that can be delivered to the horn.
Another limitation on the total energy input that can be introduced to a horn has been the limited scalability of horns. Though examples of multiple driver horns are known, typically only a single driving unit for a given frequency range has been provided. One example of a multiple driver horn (U.S. Pat. No. 5,898,138) positions a pair of low frequency transducers having throats located equidistant from the horn's mouth. While effective, such an arrangement is not readily scalable to a greater number of drivers.
In a prior application by the present inventor for a Subwoofer with Cascaded Linear Array of Drivers, U.S. patent application Ser. No. 10/649,040, filed 27 Aug. 2003, now U.S. Pat. No. 7,454,030, which is incorporated herein by reference, a folded, expanding horn loudspeaker having a selectable plurality of acoustic drivers was disclosed. The loudspeaker unit provided a compact enclosure defining the folded, expanding horn and housing the acoustic drivers. Four identical acoustic drivers were provided, each having a relatively small cone or diaphragm, and each being located in a sealed back chamber (i.e. a closed box baffle). The acoustic drivers radiated into volumetrically identical high pressure chambers located in front of the drivers. Each high pressure front chamber was coupled to a summing throat for the horn by an extended port which operated as an air pressure or air volume velocity step up transformer. The outlets of the ports were acoustically spaced from one another by equal distances and differentially spaced from the mouth of the horn. Transducer drive circuitry applied drive signals to the acoustic transducers derived from a common source. The signal to the respective acoustic transducers was delayed to compensate for the distance of the throats for the respective acoustic transducers from the mouth of the horn. The source signal was also as filtered and phase adjusted as required for clear reproduction of the sound.